With the recent spread of portable personal computers and mobile phones, the demand for secondary batteries having a higher capacity has been increasing. To meet the demand, extensive development of lithium secondary batteries having a high capacity is proceeding.
Metallic lithium and lithium alloys capable of realizing a high capacity have been used typically as a negative electrode material from the start of the development of lithium secondary batteries. Because these lithium materials form a dendrite that causes a danger of ignition or a burst, calcined carbonaceous materials capable of intercalating and desintercalating lithium have been put to practical use. However, carbonaceous materials have a disadvantage of a small discharge capacity.
JP-A-6-275268 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") and JP-A-6-338325 describe the use of SnO and the use of a composite oxide comprising tin and other elements, respectively as a negative electrode material. Some of the negative electrode materials disclosed in these publications have a high capacity but insufficient cycle characteristics. In addition, the high capacity tends to give rise to such accidents as a sudden increase in battery temperature or a spout of the contents due to abnormalities such as a short circuit. That is, safety is not ensured sufficiently.
Accordingly, an object of the present invention is to provide a nonaqueous secondary battery having a high discharge potential, a high capacity, satisfactory cycle characteristics, and excellent safety.